I am very sorry you are experiencing these thoughts that seem to prevent you from going forward. Write a list down of your currant partner of all his good attributes and focus on those. Also keeping you mind occupied with doing something that will divert you and keep mind busy. Dwelling on the past does not help. Create new memories. Stop comparing the two. People are different.

There is a story about two monks. They were by a river that needed to be crossed.
A woman came along and needed to get across as well. The first monk, we will him monk x said to the woman, "I will take you across the river". The second monk we will call monk Y said to monk x..."Our religion forbids it, you cannot take her across."
Monk X put the woman on his shoulders and carried her across. Monk Y kept chastising
Monk x........He said........"why did you carry her across?" Monk X said, "I have put her down but I see you have not dropped the pebble"

Bottom line......Let it go!

The best of luck to you and wishes for your good health.